Recovery of flux residues from rinse water in dip brazing

ABSTRACT

A process for recovery of salts from a rinse liquid solution used to rinse salt flux from a metallurgical joining operation is disclosed in which process soluble flux salts are dissolved in the rinse liquid solution while other insoluble salts are dispersed therein. The process comprises collecting the rinse liquid solution; removing undissolved solids from the solution; concentrating the remaining solution by removal of liquid to provide a slurry; subjecting the slurry to a drying operation to remove the balance of the liquid; and recovering the residue. In a preferred embodiment, the residue is analyzed and sufficient metal salts are added to restore the residue to a desired salt flux composition for reuse.

BACKGROUND OF THE INVENTION

This invention relates to salt flux and more particularly it relates torecovery of salt flux from solutions of rinse liquids used inmetallurgical joining operations such as brazing or welding of aluminummembers.

In a typical metallurgical joining operation, the aluminum parts to bebrazed are joined by an aluminum brazing filler metal, such as analuminum-silicon alloy, having a melting point slightly below thealuminum members. Brazing is effected by heating the aluminum members tobe joined and the filler metal to a temperature above the melting pointof the latter, but short of the melting point of the former. The moltenfiller metal flows and forms a joining fillet between the aluminum metalmembers. Some diffusion between parent metal and filler metal occurs andthe joint is considered, at least to some degree, to be metallurgicallybonded.

Normally, an alkali metal halide salt flux is employed to overcome theoxide coating on the parent metal which allows the filler metal to wetand flow over the parent metal surfaces. This is considered necessary toobtain the capillary flow and uninterrupted fillet formation desired. Infurnace brazing, the flux is applied as an aqueous flux suspension tothe assembled parts which are dried prior to heating to brazingtemperature. Typical flux suspensions contain 50% or more of the flux,and they deposit a substantial and easily visible amount of flux on theparts.

In dip brazing, the flux is substantially free of water and the joiningoperation takes place in the bath. However, in all of these processes,after the joining operation, residual flux salt is washed or rinsed awayfor purposes of preventing the residual flux salt from corroding thejoined members. In the rinsing operation, conventionally, the rinsewater is normally discarded when the flux salt dissolved therein reachesa certain level since, thereafter, rinsing is not efficient. Treatingthe rinse water in this way results in a substantial loss of salt fluxwhich is an economic waste as well as an ecological problem. Thus, itcan be seen that there is a great need for a system which is effectivein recovering the salt flux from the rinse water. The present inventionprovides a highly economical system for recoverying salt flux from therinse water. Furthermore, the present invention eliminates any need todump or discard the rinse water for reasons of its become ineffective.In addition, the invention can result in as much as an 80% reduction inthe loss of salt flux.

SUMMARY OF THE INVENTION

An object of the present invention is to recover salt flux used in ametallurgical joining operation from rinse liquid.

Another object of the present invention is to minimize the amount ofsalt flux lost due to rinsing in a dip or furnace brazing operation.

Yet another object of the present invention is to substantiallyeliminate dumping of the rinse water in a brazing operation.

These and other objects will become apparent from an inspection of thedrawing, specification and claims.

In accordance with these objects, there is provided a system forrecovery of solids from a rinse liquid solution used to rinse salt fluxfrom a metallurgical joining operation wherein soluble flux salts aredissolved in the rinse solution while other insoluble salts aredispersed therein. The system comprises the steps of collecting therinse liquid solution; removing undissolved solids therefrom; andconcentrating the remaining solution by removal of liquid to a slurrycontaining at least 30 wt.% solids. The slurry is then subjected to adrying operation to remove the balance of the liquid, and the residue isrecovered. Operation of the system in accordance with the invention canresult in as much as 80 wt.% of the flux removed in the rinse beingrecovered. In a preferred embodiment, the residue is analyzed andsufficient metal salts are added to restore the residue to a desiredsalt flux composition for reuse. Thus, the amount of metal salt requiredto maintain the flux in active condition is lowered considerably.

DESCRIPTION OF THE INVENTION

Referring now to the flow chart, box 2 (labelled "Flux Source") may be abrazed or welded article having flux residues remaining thereon.Typically, the amount of flux residue remaining on the brazed article ison the order of about 0.5 ozs/ft² aluminum. Although the amount canincrease in the case of aluminum radiators, for example, where, becauseof the fins substantial capillary action can be encountered. On thebasis of pounds of aluminum brazed, the amount of flux removed from theflux bath, which in turn is removed by the rinse, is on the order ofabout 0.2 lbs/lb of aluminum. It will be appreciated that this amountcan vary. While these numbers are provided to demonstrate the brazingprocess, they also provide some indication of the amount of flux lost inthe rinsing process and the economic benefits to be derived from theinstant process. Further, any problems with respect to dispersion of therinse water are virtually eliminated from an environmental viewpoint.Rinse liquid such as water from rinse tank 4 is applied to rinse theflux from the article. The water may be applied by spraying or dipping,and while only one tank is represented, there may be a number of tankshaving increasing purity with respect to the water. The flux salts arethen either dissolved into the rinse water, or suspended or dispersedtherein if they are insoluble. It should be recognized here that therinse liquid will most probably consist of ordinary water although otherliquids, such as dilute acids, could be used. Furthermore, whileordinary tap water can be used, it may be preferable to provide someacidity in the water such as by the addition of nitric acid to aid indissolution of particular flux salts. Typically, the pH is not normallylower than 2.0 and is generally maintained in the range of about 2.5 to3.5.

The rinse liquid may be applied continuously until the accumulation ofdissolved and suspended salts interferes with satisfactory rinsing, or,alternatively, a portion of the rinse liquid may be continuouslywithdrawn for processing in accordance with the invention.

The rinse liquid to be processed is transferred to settling tank 6 whereundissolved salts are permitted to settle out. The supernatant liquid isthen passed through filter 8 which removes most of the remainingsuspended solids as sludge as shown in the flow sheet. It will beunderstood that any of these steps may be performed on a continuousbasis.

The filtered liquid is then subjected to one or more steps toconcentrate the liquid for eventual recovery of the dissolved saltstherein. As shown in the flow chart, the filtered liquid may optionallybe put through a reverse osmosis stage wherein the solution of rinseliquid and dissolved salts is concentrated while purified liquid, e.g.water, is removed and may be recycled back to rinse tank 4 via holdingtank 12.

The filtered liquid is then subjected to an evaporation step 14, which,as alluded to above, may be used either in place of or as a supplementto the reverse osmosis step. Evaporation step 14, at its simplest, maycomprise an open vessel to which heat is applied, whereby the liquid,e.g. water, boils off leaving the salt residue behind. That is, thepurified water is not collected. The evaporation step may comprise amore sophisticated apparatus where the liquid is not only removed fromthe salt residue but is collected and recycled back to rinse tank 4.Such a system is referred to as the Alcoa Thermopure System, which isdescribed and claimed in Brown U.S. Pat. No. 3,843,463, issued Oct. 22,1974, and incorporated herein by reference. In the evaporative step, thesalts should not be concentrated to provide a slurry having greater thanabout 50 wt.% solids, and perferably, the solids should not beconcentrated to a level greater than 30 wt.% in order to avoidinterferring with operation of the system.

The evaporated slurry or residue may then be removed from the evaporator14 and placed in a holding tank 16 prior to placement in dryer 18wherein the residue is heated to a temperature sufficient to remove asubstantial amount of liquid, e.g. water. This may be accomplished byheating it to a temperature of about 110° to 150° C. for about 30minutes. If it is desired to use the salts in the flux bath again, thenthe water content remaining therein should not be greater than 0.5 wt.%and preferably not greater than 0.3 wt.%. It is very important that thewater level be carefully controlled to avoid introduction of water tothe flux bath where it can operate to oxidize the aluminum surface asopposed to removing the oxide therefrom.

The dried salts removed from the dryer at 20 are preferably disposed ofby recycling for further use as flux agents. For recycling at 22, theresidue is analyzed to determine the chamical balance of theconstituents to determine what salts have been lost during the process.Sufficient additional salts are then added to provide the desiredconcentration and ratios of salts.

It should be noted here that the term "salts" as used herein is used ina broad sense to encompass not only metal halides, but other metalcompounds encountered in the rinse water.

The invention is most effective when the flux ingredients are recycledand reconstituted by the addition of makeup ingredients for those lostand is most useful when a flux is used, the salts of which are solublein the rinse water. The loss of some flux ingredients is not simply dueto inefficient recovery procedures, but may be due to chemicalconversions occurring during application and usage of the flux whereinsoluble flux ingredients interact to form insoluble compounds or reactwith the oxides on the metals to be metallurgically joined together toform insoluble residues.

For example, the present invention is not intended for use with fluxescomprising the fluoride type salts, such as the soduim, potassium,calcuim fluorides or cryolite salts. That is, the system is not intendedto recover these types of salts from water. Rather the present inventionis intended to be used to recover fluxes whose constituents are to alarge extent soluble in the rinse water.

For example, the joining of aluminum or aluminum alloy parts by dipbrazing involves the use of a molten combination of halides, includingalkali metal chlorides, such as LiCl, NaCl and KCl as well as alkalineearth chlorides, such as CaCl₂ with metal fluorides such as AlF₃, NaF orthe like. The use of NaF in such a combination has been found to resultin the formation of CaF₂ residue which is not soluble in the rinsewater, and, therefore, such flux would normally not be consideredsuitable for use with the present invention. The CaF₂ residue forms inthe flux when it is held in a molten condition for an extended period oftime, i.e. over 24 hours. In contrast, the system of the invention hasbeen found to function well with the use of a CaCl₂, NaCl, KCl and AlF₃flux wherein the chloride component is controlled to 27.5 to 32.0 wt.%CaCl₂, 13.5 to 22.0 wt.% NaCl and 39.2 to 51.0 wt.% KCl and AlF₃ isadded to constitute 5 to 8 wt.% of the total flux (described and claimedin copending application Ser. No. 40,157, entitled Dip Brazing Flux).Other fluorides which may be used alone or in conjunction with AlF₃include soduim, potassium and lithium florides. In the use of theaforesaid flux in the joining together of aluminum alloy members, theflux ingredients dissolve in the rinse water while insoluble materialssuch as Al₂ O₃ are removed in the initial settling. The flux can then berecycled for use by addition of sufficient makeup salts to restore thesalts in condition suitable for adding to the flux bath.

The following examples are further illustrative of the invention.

EXAMPLE 1

An aluminum flux bath was made up comprising, by weight, 28.81% NaCl,48.36% KCl, 16.28% LiCl and 6.55% AlF₃ and aluminum assemblies werebrazed therein at a temperature of 602° C. The assemblies used a baseplate of #23 brazing alloy (Aluminum Association designation) andaluminum members of 3003 alloy (Aluminum Association designation) whichwere joined during the brazing operation. The brazed assemblies werethen rinsed in deionized water at room temperature. The rinse water,which contained about 3 wt. % soluble compounds, was first filtered toremove insoluble compounds, such as Al₂ O₃ and insoluble fluorides, andthen treated in accordance with the Thermopure process (referred toearlier) and the soluble salts were concentrated to about 30 wt. %. Thesalt slurry was dried in an oven at a temperature of 125° C. The watercontent thereof was about 0.5 wt. %. The dried salts were then analyzedand reconstituted by adding thereto 6.90 gms KCl, 1.66 gms NaCl, 0.00gms LiCl and 7.55 gms AlF₃, which was addition of 16.11 gms of new saltsper 100 gms of recovered salt. Thus, using the reconstituted salt whichhad a composition, by weight, of about 50.5% KCl, 28.0% NaCl, 15.0% LiCland 6.5% AlF₃, assemblies, as mentioned before, were brazed and brazefillets which were found to be essentially complete, e.g. greater than90%. Using an aluminum base plate of #23 alloy, the fillets were 100%complete for three samples. This compared very favorably with assembliesbrazed in the original flux where the fillets were 88.7% complete for#11 alloy and 96.3 complete for #23 alloy.

EXAMPLE 2

An aluminum flux brazing bath was made up containing, by weight, 31.5%CaCl₂, 17.9% NaCl, 44.1% KCl and 6.5% AlF₃ and assemblies were brazedtherein as in Example 1. Also, the rinse water used was collected andtreated as in Example 1. The dried sample was analyzed. 18.74 gms of amixture containing 37.0% CaCl₂, 21.9% KCl and 41.1% AlF₃ were added toeach 100 gms of the recovered salts to provide a flux compositionsubstantially the same as the starting composition. The reconstitutedflux was used for brazing assemblies as in the original bath. It wasfound that using #23 alloy as the base plate, 100% fillets were obtainedand using #11 alloy as the base plate, 85% or better fillets wereobtained which compared very favorably with fillets obtained in theoriginal bath.

EXAMPLE 3

An aluminum flux brazing bath containing, by weight, 23.0% CaCl₂, 49.0%KCl, 17.7% NaCl, 6.1% LiCl, 1.9% AlF₃ and 2.3% NaF₃ was used to brazeassemblies as referred to in Example 1. High quality fillets wereobtained with 90% or more of the joint being brazed. The assemblies wererinsed and the rinse water treated to recover the dissolved salts. Thedried salts obtained were analyzed and 46 gms of a mixture containing15.1% CuCl₂, 20.5% KCl, 30.0% LiCl, 2.2% AlF₃ and 3.2% NaF was added toevery 100 gms of dried salts to return the flux to what was believed wasthe original composition. The flux was then used for brazing assembliesas above. However, only about 20% of the joint area had been brazed, andit was found that the samples had become badly discolored. The reasonsfor the failure of the flux in this example are not fully understood butit is believed that a compound such as CaCl₂.Ca(OH)₂ is formed which isnot believed to be formed in the aforementioned examples. It is believedthat such compound does not undergo ionization in the molten flux toform chloride ionic species and thus acts to inhibit the brazing action.

These examples demonstrate that flux salts can be recovered from rinsewater and can be returned to a state whee they can be reused in a saltflux thereby greatly conserving the amount of salt required for theflux.

While the invention has been described with respect to certainembodiments, other modifications will be apparent to those skilled inthe art which are within the scope of the invention, as defined in theappended claims.

What is claimed is:
 1. A process for recovery of fluxing salts from arinse liquid solution used to rinse salt flux from an aluminum joiningoperation wherein soluble flux salts are dissolved in the rinse liquidwhile other insoluble salts are dispersed therein which comprises:(a)providing a salt flux for joining an assembly of aluminum members; (b)submerging the assembly in the salt flux; (c) removing the assembly fromthe flux and treating it with rinse liquid to remove flux adheringthereto; (d) collecting rinse liquid solution; (e) removing undissolvedsolids from the solution; (f) concentrating the remaining solution byremoval of liquid; (g) drying the concentrated solution; (h) recoveringa salt residue from the concentrated solution; (i) analyzing the saltresidue and adding flux salt to provide a reconstituted residue suitablefor use with the original salt flux; (j) adding the reconstituted saltflux to the original salt flux; and (k) brazing aluminum assembliestherein and obtaining high quality brazed joints in said assemblies. 2.The process according to claim 1 wherein the flux contains metalchlorides and metal fluorides, the amount of fluoride present in theflux being controlled so that the salts have a high solubility in therinse water.
 3. The process according to claim 1 wherein the dip brazingflux contains CaCl₂, NaCl, KCl and at least one metal fluoride selectedfrom the group consisting of sodium, potassium, lithium and aluminumfluoride and is characterized by a low formation of CaF₂ when the fluxis maintained in a molten condition for an extended period of time. 4.The process according to claim 3 wherein the amount of CaF₂ formed isnot greater than about 5.0 wt.% of the flux.
 5. The process according toclaim 3 wherein the flux contains aluminum fluoride.
 6. The processaccording to claim 1 wherein the flux contains 13.5 to 22.0 wt.% NaCl,39.2 to 51.0 wt.% KCl and 27.5 to 32.0 wt.% CaCl₂ based on the chloridecomponent.
 7. A process for recovering flux salts from a rinse solutionused for rinsing salt flux from aluminum members joined in a dip brazingoperation, the process comprising the steps of:(a) providing a moltenflux containing NaCl, KCl, CaCl₂ and at least one fluoride salt selectedfrom the group consisting of sodium, potassium, lithium and aluminumfluoride, the flux characterized by formation of not greater than 5.0wt.% CaF₂ after standing for a period of 8 hours in the moltencondition; (b) dipping the aluminum members therein for joining; (c) onremoval of said members from the flux, treating them with rinse solutionto remove residual flux adhering thereto; (d) collecting the rinse waterand removing undissolved solids therefrom; (e) concentrating thedissolved salts by removal of liquid to provide a slurry having notgreater than 50 wt.% solids; (f) subjecting the concentrated salts to adrying action to provide salts having a liquid content not greater than0.5 wt.%; and (g) analyzing said dried salts and adjusting thecomposition thereof for use in the molten flux.
 8. A process forrecovering flux salts from a rinse solution used for rinsing salt fluxfrom aluminum members joined in a dip brazing operation, the processcomprising the steps of:(a) providing a molten flux containing NaCl,KCl, CaCl₂ and at least one fluoride salt selected from the groupconsisting of sodium, potassium, lithium and aluminum fluoride, the fluxcharacterized by formation of not greater than 5.0 wt.% CaF₂ afterstanding for a period of 8 hours in the molten condition; (b) dippingthe aluminum members therein for joining; (c) on removal of said membersfrom the flux, treating them with rinse solution to remove residual fluxadhering thereto; (d) collecting the rinse water and removingundissolved solids therefrom; (e) concentrating the dissolved salts byremoval of liquid to provide a slurry having not greater than 50 wt.%solids; (f) subjecting the concentrated salts to a drying action toprovide salts having a liquid content not greater than 0.5 wt.%; (g)analyzing said dried salts and adjusting the composition thereof for usein the molten flux; and (h) adding the adjusted salt flux to theoriginal molten flux and brazing aluminum members therein to obtain highquality brazed joints therebetween.